3 ROS2 Topics

Open a new terminal (Ctr + Alt + T) and run:

ros2 run turtlesim turtlesim_node

Open another terminal (Ctr + Alt + T) and run:

ros2 run turtlesim turtle_teleop_key

Open another terminal (Ctr + Alt + T) and run:

rqt_graph

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Running the ros2 topic list command in a new terminal (Ctr + Alt + T) will return a list of all the topics currently active in the system:

ros2 topic list

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To see the data being published on a topic, use: ros2 topic echo <topic_name> in a new terminal (Ctr + Alt + T)

ros2 topic echo /turtle1/cmd_vel

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• Another way to look at this is running in a new terminal (Ctr + Alt + T)

ros2 topic info /turtle1/cmd_vel

ros2 interface show

a new terminal (Ctr + Alt + T), run this

ros2 interface show geometry_msgs/msg/Twist

This expresses velocity in free space broken into its linear and angular parts.
Vector3 linear
Vector3 angular

ros2 topic pub

Now that you have the message structure, you can publish data onto a topic directly from the command line using:

ros2 topic pub --once /turtle1/cmd_vel geometry_msgs/msg/Twist "{linear: {x: 2.0, y: 0.0, z: 0.0}, angular: {x: 0.0, y: 0.0, z: 1.8}}“

--once is an optional argument meaning “publish one message then exit”.

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ros2 topic pub --rate 1 /turtle1/cmd_vel geometry_msgs/msg/Twist "{linear: {x: 2.0, y: 0.0, z: 0.0}, angular: {x: 0.0, y: 0.0, z: 1.8}}“

The difference here is the removal of the --once option and the addition of the --rate 1 option, which tells ros2 topic pub to publish the command in a steady stream at 1 Hz.

Ros2 topic echo

To see the message

ros2 topic echo /turtle1/pose

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To see pub rate

ros2 topic hz /turtle1/pose

average rate: 59.354
min: 0.005s max: 0.027s std dev: 0.00284s window: 58

Reference : https://docs.ros.org/en/humble/Installation.html